Recently, as the weight of the renewable energy is increasing, power storage devices are drawing attention as a new alternative which can overcome the problems of flexibility in power production and misalignment in the timing of seasonal demand and supply. Accordingly, research and development on power storage devices are actively in progress.
The secondary batteries for large capacity power storage are lead storage battery, NaS battery, redox flow battery (RFB), and the like.
Especially the redox flow battery has features in that the maintenance cost is low while it is operable at room temperature, furthermore, the capacity and the output can be designed independently. So many researches of the redox flow battery are in progress for a large capacity secondary battery.
Generally, a redox flow battery, as illustrated in FIG. 1, includes: a positive cell 210; a negative cell 220; a separating membrane 230, formed between the positive cell 210 and the negative cell 220; a positive electrolyte tank 280, wherein positive electrolyte is stored for supplying positive electrolyte to the positive cell 210 by driving the pump 281; and a negative electrolyte tank 290, wherein negative electrolyte is stored for supplying negative electrolyte to the negative cell 220 by driving the pump 291.
The positive cell 210 and the negative cell 220 may be stacked in multiple layers, and a current collector and an end plate are disposed at the outer sides of the outermost positive cell 210 and the outermost negative cell 220.
Generally, each of the positive cell 210 and the negative cell 220 includes a felt electrode, a bipolar plate, and a manifold containing electrolyte flow path.
In the redox flow battery, the electrolyte includes a redox couple, and the reduction oxidation reaction occurs in the positive cell 210 and the negative cell 220 according to charging and discharging.
For example, when a vanadium couple is used as redox couple, the reactions occurring at the positive cell 210 and the negative cell 220 are as follows.Positive electrode: V4+→V5++e−(charging)V4+←V5++e−(discharging)Negative electrode: V3++e−→V2+(charging)V3++e−←V2+(discharging)  <Reaction equation 1>
Generally, the redox flow battery of this structure is used in the factories and the like, as an emergency power source in a form wherein the positive cell 210 and the negative cell 220 are multiply stacked.
Since the redox flow battery, which is used as an emergency power source, is being kept in a charged state and operated only in an emergency situation, may have a long standby time.
Thus, when the standby time of the redox flow battery is getting longer, the shunt current is generated.
In other words, when the standby time of the redox flow battery is getting longer, self-discharge is occurring since a electrochemical reaction is generated as the active materials, which are dissolved in the electrolytes existing inside of the positive cell 210 and the negative cell 220 of the stack, are moving towards the opposite side through the separating membrane 230.
In addition to this, more self-discharge is occurring when the massive amount of electrolytes remaining in the pipes 282, 292 and the like which are the flow paths of the electrolytes installed for supplying electrolytes to the positive cell 210 and the negative cell 220.
Since energy must be applied in order to recharge the electrolytes discharged in such a way, there is no way to avoid a power loss.
In order to avoid occurrence of a shunt current in a way as described above, when a redox flow battery is installed, the pipes 282, 292 and the electrolyte tanks 280, 290 are positioned low such that the remaining electrolytes inside the stack or the pipes 282, 292 and the like can be moved towards the electrolyte tanks 280, 290 when the operation is stopped.
However, when the pipes 282, 292 and the electrolyte tanks 280, 290 are installed in a low position, there are problems in that the embrittlement may be weakened since the separating membrane 230, which is existing inside the stack, may be kept dried. Moreover, more or less longer time is required for charging and discharging since charging and discharging starts only when the electrolytes stored in the electrolyte tanks 280, 290 are supplied into the stack using the pumps 281, 291 when charging and discharging (the battery).